1. Field of the Disclosure
The present disclosure relates in general to the field of acoustic testing in a downhole environment and provides an acoustic source suitable for use in acoustic tests in a wireline or logging-while-drilling device.
2. Description of the Related Art
In petroleum exploration, acoustic testing is often used to obtain information about formations surrounding a borehole. Typically, one or more acoustic sources are generally conveyed downhole on a wireline or logging-while-drilling (LWD) device, for example. The one or more acoustic sources are activated from a downhole location to produce one or more acoustic pulses. An acoustic response of the surrounding formation to the one or more acoustic pulses are then recorded and analyzed. Typically downhole testing requires an acoustic source that has an operating acoustic range that extends from several hundred Hertz (Hz) at a low frequency end to a few kHz at a high frequency end.
Numerous borehole acoustic testing applications typically require a compact, efficient, low-frequency acoustic source. Electrical power is generally limited for downhole tools in logging-while-drilling (LWD) measurements. Thus electrical efficiency of electro-acoustic conversion of acoustic tools is generally desired. Also, the radial depth that is available for mounting individual source elements on such a downhole conveying device is typically limited due to requirements for overall strength of the drill collar in the face of local stresses. Complex configurations, such as an acoustic quadrupole configuration in which four acoustic sources are required at the same general axial location, further limit on the amount of available radial depth. Additionally, LWD quadrupole measurements use a certain bandwidth of a generated low frequency acoustic signal. Thus, a selected frequency region is desired from the downhole acoustic source.
Various materials are currently being used in acoustic sources used downhole. One of the most commonly used materials is piezoceramics. Piezoceramic acoustic sources provide good performance for high frequency applications (i.e., in a range of several kHz to a few hundred kHz). However, these sources tend to lack efficiency at lower frequencies. Alternatively, other acoustic sources use magnetostrictive materials. In contrast to piezoceramic acoustic sources, magnetostrictive sources provide efficiency at low frequencies (around 1 kHz). However, these sources generally require a fairly long active element or force direction converter in order to provide an adequate force/displacement at an active acoustic radiator (i.e., piston or membrane). The size of the active element/force direction converter is problematic when attempting to fit the source into an available space within a drill collar. Also, configurations of multiple acoustic sources facing in different azimuthal directions, i.e. quadruple configurations, are especially sensitive to alignment. The presence of a converter typically makes alignment difficult.
Therefore, there is a need for an electromagnetic linear acoustic source that can provide acoustic pulses over a suitable acoustic frequency range and operate from a device downhole, such as a LWD or wireline system.